Hi and welcome to Psych Central, hope you find it a helpful and supportive community.

It sounds like you have been living with a lot of instability and drama for a long time now, and so your kids have too. I'm not sure taking up with a girlfriend on and off is going to help the situation which already sounds stressful for all concerned. In fact I would be very wary of expecting her to be your 'saviour', that is a tall order on any relationship.

Your wife's well-being is off course important to your kids well-being, and something you need to consider, that does not necessarily mean staying married if you feel the marriage is no longer working however. If you do feel divorce is the best option then you can work to make sure the disruption is minimised and your kids are fully supported by you (and hopefully their mother). It doesn't have to be the worst case scenario.